This invention relates to printers employing a movable carriage and more particularly it concerns a resilient mounting arrangement for a lead screw or other such rotary drive shaft, particularly as used in a carriage drive system.
Matrix printers of the type having a movable, carriage-supported, multiple solenoid-type print head are known in the prior art. The carriage and print head are combined with a conventional platen, paper advance and a logically controlled actuation system to permit matrix printing, which may be done in both a forward and reverse direction. Such printers are commonly employed as readout terminals for digital computer systems.
Conflicting requirements are encountered in the selection and in the design of a data printer. In the environment of a computer system, the speed at which information can be obtained is generally limited by the speed of the printer employed as an output terminal. However, as more sophisticated drive systems are employed to increase the output of the printer, the cost to manufacture and purchase such a printer also drastically increases. In an attempt to maintain an adequate print rate while maintaining the costs of manufacture and therefore the price of the printer within a reasonable range, impact matrix printers had been proposed which employ a lead screw drive shaft upon which the print head carriage is threadably disposed. In such an arrangement, a pair of supporting end plates or like members are provided. A guide or slider rod and the lead screw shaft extend longitudinally between the end plates. The carriage structure includes slide bearings for slidably securing the carriage to the guide rod. The support end plates and guide rod must provide a rigid support system so that a fixed, predetermined distance and position relationship is always maintained between the print heads and the platen during operation of the high speed printer, since this is essential to maintain printing operation of the device.
Certain problems are encountered, however, with such a drive train and support structure due to the fact that the screw shaft is constantly being abruptly started and stopped, rotating and reversing its direction of rotation, at a rapid rate. The lead screw is thus subjected to alternating torsional stresses and it is also under axial loading due to the alternating motion of the carriage and print head structure. Further, due to variances in manufacturing tolerances in units of commercial quality the lead screw is likely to be bowed or irregularly non linear in shape, in varying degrees, and may be further distorted by stresses incurred during operation. Also, during operation of the screw shaft drive system, the lead screw, being by nature a mechanism of limited mechanical efficiency, is subject to heating, causing thermal expansion. As a result, the lead screw tends to bind after a period of initial use, further aggravating the above-noted problems. In fact, the lead screw may jam, and the internally threaded follower portions of the carriage which ride on the screw shaft may become jammed, or damaged, thereby severely impairing the overall operation of the printer and perhaps causing malfunction.